Research for the method of constructing velocity field in a complex area is a frontier research subject that long exists in petroleum geophysical exploration, and has not been completely solved. Currently, many theoretical and practical application achievements have been obtained, but the research mostly focuses on the following two aspects:
(1) Research on how to obtain more accurate average velocity field with seismic data
The research mainly includes two aspects: firstly, improving calculation accuracy of iterative velocity, with methods of iterative weighted complex trace velocity analysis, iterative weighted inclination velocity analysis, etc.; secondly, improving calculation accuracy of interval velocity, with methods such as generalized Dix method and various velocity smoothing methods, etc. Since there are many factors that affect seismic velocity, the above methods though improve accuracy of velocity analysis to some extent, but cannot guarantee accurate average velocity field.
(2) Comprehensive study on average velocity field in accordance with seismic, logging and VSP data
At present, said methods are main hotspots of research, and have achieved great research results and relatively wide applications. However, the research of such aspect mainly depends on an interpreter's experience, summarizing seismic velocity, logging and VSP data, which requires very heavy human workload. In addition, different interpreters would obtain seismic velocity field differing greatly from their own experience, and it is difficult to accurately constructing a subterranean velocity field.
The propagation velocity of a seismic wave in strata is related to factors of lithology of rock, porosity, burial depth, pore-fluid property, pore fluid pressure and external environment of the strata, and the like, wherein the lithology is the most important factor. Strata of different eras present difference in terms of lithology and porosity, and thus in terms of velocity. Generally, from new eras to old ones, the interval velocity of a stratum gradually increases.
In strata of the same era, particularly in marine bed, the lithology is common Generally, transverse variation of velocity is caused by two factors. One is transverse variation of sedimentary facies, resulting in the lithology changes transversely, so that the interval velocity also changes transversely. Theoretically, the interval velocity of a single compact rock is close to a determined value, but actual strata are not composed of “single” component, but formed by mixing different components, such as argillaceous sandstone, calcareous sandstone, and the like, which makes the velocity of a seismic wave have mixed transitional nature, depending on contents of various components. Besides, porosity, style of pore fillings and pore fluid pressure and the like also affect velocity. The other factor affecting the transverse variation of velocity is depth (pressure). Generally, the deeper burial, the more compact, the smaller porosity, and the faster velocity.
It is assumed that in the case the burial depth is the same, and the temperature, load pressure and the like conditions are the same or similar, relationship between velocity and depth, i.e. a compaction curve, can be calculated by a statistical method. When taking interval velocity of data of the respective measured points in a certain terrane as the X-axis, a depth thereof as the Y-axis, a normal velocity-depth curve (i.e. H-V curve) is obtained by fitting tendency that the velocity varies with the depth of the scattered points. Generally, the variation of velocity with depth can be described by a function of first degree, that is, the velocity is deemed to increase linearly with depth, represented by the following formula:V=ah+b   (1)
wherein v is interval velocity, h is strata burial depth, and a, b are characteristic parameters of the velocity-depth curve (i.e. H-V curve).
Since there are lots of methods for acquiring subterranean velocity, there are lots of methods for acquiring a velocity-depth curve. For example, an interval velocity-depth curve can be obtained according to logging velocities of multiply wells; a velocity-depth curve can be obtained according to conversion interval velocities of multiple wells; or an interval velocity-depth curve can be obtained according to seismic velocity. Since there are many factors that affect the seismic velocity and the conversion velocity, among the plurality of curves, the interval velocity-depth curve obtained according to the logging interval velocities of multiple wells can best represent real subterranean conditions.
As mentioned above, the variation of velocity with depth can be described with a function of first degree, then whether the one velocity-depth curve can describe transverse velocity variation in a survey or not? The answer is no. This is because there are two main factors affecting transverse variation of velocity. The tendency that velocity changes with depth (pressure) can be described by value a of the above-mentioned velocity-depth curve. In a relatively small survey, the value a in the velocity-depth curve is relatively constant, particularly in the marine bed, the value a is almost constant. Thus in a relatively small survey, a fixed value a can be used to describe the tendency that velocity changes with depth (pressure). However, in a relatively large survey, the value a should be variable.
The other factor prominently affecting seismic interval velocity is transverse variation of sedimentary facies. Theoretically, the interval velocity of a single compact rock is close to a determined value, actually, strata are not composed of “single” component, but formed by mixing different components, such as argillaceous sandstone, calcareous sandstone, and the like, which makes the velocity of a seismic wave have mixed transitional nature, depending on contents of various components. Even if in a marine bed with relatively stable sedimentary facies, its ingredients are still different, but the velocity varies less relative to land strata. Besides, porosity, style of pore fillings and pore fluid pressure and the like also affect velocity. If the same function of first order is used to describe transverse variation of seismic velocity, the result is, even in a relatively small survey, the variation of value b is very large.
A basic conclusion is obtained from the above discussion, that is, variation of velocity with depth can be described with a function of first order, and the H-V curve varies transversely, but the variation tendency of value a and that of value b in the curve are different, wherein the value a is relatively constant, varies little transversely; whereas the value b is affected by many factors, varies violently transversely.
The above basic conclusion is commonly accepted, but the key point is how to apply the basic conclusion in practical complex velocity construction, i.e. how to construct a transverse variable H-V curve in the practical velocity construction. Especially, result of the complex velocity construction is generally to be used in practical time-depth conversion, the result of which should correspond to practical drilling result; therefore, construction of a transverse variable H-V curve has to be constrained by the drilling result.